jeffrey brown has that part of the story. >> brown: the syrian civil war has now closed in on president bashar al-assad's seat of power, with rebel gunfire ringing out again today in damascus. amateur video also showed new shelling in the capital, as government forces continued a push to retake key suburbs. and while the noise of war grows louder in damascus, so have fears around the world that assad may resort to chemical weapons. the syrian government has a number of sites containing what may be the largest chemical weapons stockpile in the world. it's made up largely of sarin nerve gas, mustard gas and cyanide. on monday, president obama sounded a warning, amid reports of unusual activity at the weapon sites. >> the world is watching. the use of chemical weapons is and would be totally unacceptable. if you make the tragic mistake of using these weapons there will be consequences and you will be held accountable. >> reporter: assad's regime answered that it would never use such weapons against its own people, but today, at nato headquarters in brussels, secretary of state hillary clin